1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus capable of recording information signals on an optical recording medium, such as an optical disk and/or reproducing information signals from the optical recording medium and, more particularly, to an objective lens driving device capable of efficiently and quickly correcting the tilt of an objective lens relative to a signal recording surface of an optical disk so that comatic aberration of a spot of a light beam formed on the signal recording surface of the optical disk is reduced, and an objective lens support device.
2. Description of the Related Art
An optical disk drive, namely an apparatus which projects a light beam on the signal recording surface of a rotating optical disk to read information signals recorded on the optical disk or to write information signals to the optical disk, employs an optical head as a means for writing (recording) and reading (reproducing) information signals. The optical head comprises, as principal components, a light source, such as a semiconductor laser, optical elements including a collimator lens, a beam splitter, an objective lens and a photodetector. A light beam emitted by the light source is guided by the optical elements including the collimator lens and the beam splitter to the objective lens, the objective lens focuses the light beam on the recording surface of the optical disk, a light beam reflected from the recording surface is detected by the photodetector for recording and reproducing information signals.
An objective lens driving device drives the objective lens by an electromagnetic driving force to move the objective lens in directions parallel to two axes, i.e., a direction parallel to the axis of the light beam (focusing direction), which will be referred to as "beam axis", and a direction perpendicular to the beam axis (tracking direction). Thus, the light beam is focused on the recording surface of the optical disk and is moved correctly for scanning recording tracks.
In either an integrated optical head integrally provided with an optical system or a sectional optical head having a fixed optical system and a movable objective lens, an objective lens driving device is able to move the objective lens by about .+-.0.6 mm along the focusing direction and about .+-.0.4 mm along the tracking direction.
Generally, optical disks are resin moldings of a polycarbonate resin or an acrylic resin and a subject to static warping after being molded. If the optical disk has a warp, pit surfaces slope and thereby a reflected light beam travels at an angle to a projected light beam. Therefore, if the angle between the reflected light beam and the projected light beam exceeds an allowable limit, the reflected light beam falls outside the field of the objective lens and hence an output corresponding to the intensity of the reflected light beam is reduced. If the pit surface slopes, a working track pitch between the adjacent tracks is reduced accordingly. Accordingly, if the diameter of a beam spot formed on the pit surface is fixed, information reproduced from pits of a track is liable to be affected by information recorded on pits of an adjacent track and the intensity of crosstalk increases.
A known reproducing apparatus proposed to prevent the deterioration of reproducing performance is provided with a tilt regulating servomechanism which turns an optical system including an objective lens in a plane including the objective lens according to the static warp of an optical disk so that the optical axis of the objective lens is always perpendicular to the pit surface. The reproducing apparatus provided with such a tilt regulating servomechanism is able to regulate the tilt of the objective lens only according to a static warp.
An optical disk drive disclosed in JP-A No. 7-65397 (cited reference) is capable of preventing the deterioration of signal quality due to the tilt of the beam axis of a light beam relative to the recording surface of an optical disk and of quickly correcting the tilt of the beam axis of the light beam relative to the recording surface of the optical disk varying according to the variation of the warp of the optical disk in one turn of the optical disk. In the optical disk drive disclosed in this cited reference, an objective lens holder holding an objective lens is supported by four flexible wires so that the objective lens holder can be turned in the directions of the focusing direction, the tracking direction and a direction in which the optical axis of the objective lens is tilted. A plurality of coils for focusing, tracking and tilting are attached to the objective lens holder.
Such a mechanism of a four-wire actuator system shown in the cited reference inevitably increases the dimensions of the objective lens lens holder, i.e., a movable unit, and hence an objective lens driving device cannot be formed in a small thickness. In the mechanism of a four-wire actuator system shown in the cited reference, the center of radial tilt turning of the objective lens holder and the center of tangential tilt turning are dislocated greatly from the principal point of the objective lens. Therefore, the objective lens is dislocated from the beam axis of the incident light beam in proportion to the distance between the center of turning of the objective lens holder and the objective lens if the objective lens moves in the tilting direction of the beam axis of the light beam.
In the mechanism of a four-wire actuator system shown in the cited reference, it is impossible to make the center of gravity of the objective lens holder and the center of tangential tilting of the objective lens holder coincide with each other, cross action resonance is liable to occur and auxiliary resonance occurs in frequency characteristics with respect to the directions of actions of the actuator.
Furthermore, the mechanism of a four-wire actuator system shown in the cited reference needs an additional damping mechanism for attenuating the amplitude level of the auxiliary resonance occurred in frequency characteristics with respect to the directions of actions of the actuator, which makes the miniaturization of the objective lens driving device and the reduction of the cost of the objective lens driving device difficult.